Retention of conditioned noise aversion following medical geniculate lesions in the rat

Abstract

The importance of upper auditory pathways at the medial geniculate level was examined in relation to behavior maintained by the termination of intense noise. Rats were conditioned to escape white noise by depressing a lever. Experimental animals were then subjected to varying lesions of the medial geniculate nuclei or posterior thalamic region, with or without lesions of the brachium of the inferior colliculus. Destruction of the medial geniculate nuclei or posterior thalamic region bilaterally did not result in loss of the conditioned behavior. More than half of these animals showed significant postoperative behavioral improvements which were rarely matched by the control animals. Postoperative removal of the noise contingencies to responding was followed by behavioral extinction. Lay-off periods up to 40 days or subjection to anesthesia failed to produce any significant behavioral alterations. However, bilateral tympanic destruction produced definite losses in the response to noise. The evidence suggested that geniculocortical connections are not essential to noise aversion. One case with additional midbrain damage supported earlier findings that this region may be more critical for noise aversion. The possibility that reciprocal (efferent) auditory pathway damage could be responsible for the behavioral improvements is discussed.